Thick film dielectric structures as described in U.S. Pat. No. 5,432,015 (the disclosure of which is incorporated herein by reference in its entirety) provide superior resistance to dielectric breakdown as well as a reduced operating voltage as compared to thin film electroluminescent (TFEL) displays. The thick film dielectric structure as deposited on a ceramic substrate will withstand somewhat higher processing temperatures than TFEL devices, which are typically fabricated on glass substrates. This increased high temperature tolerance facilitates annealing of phosphor films at higher temperatures to improve their luminosity. However, even with this enhancement, it is still desirable to improve display luminance and colour co-ordinates to keep pace with ongoing improvements to cathode ray tube (CRT) displays, particularly with recent trends in CRT specifications to higher luminance and higher colour temperature.
A high luminance blue-emitting electroluminescent phosphor is desirable in electroluminescent colour displays to achieve adequate luminosity. Blue light emitting phosphor materials such as activated barium thioaluminates provide excellent blue colour co-ordinates and higher luminance, but as a ternary compound, its stoichiometry is also somewhat difficult to control. Vacuum deposition of barium thioaluminate phosphor films comprising this material from a single sulfide source pellet using sputtering or electron beam evaporation has yielded films with adequately high luminosity.
A useful approach that has been adopted to control the stoichiometry of thioaluminate phosphors is to use two separate sources for deposition. This approach requires added controls over the relative deposition rates for the different sources. U.S. Pat. No. 6,447,654 discloses the sputtering of barium thioaluminate phosphor films from a single target comprising aluminum sulfide and barium sulfide and optionally magnesium to deposit blue-emitting barium thioaluminate or barium magnesium thioaluminate phosphor materials. The stoichiomentry of the deposited film is adjusted by adjusting the target composition to account for differential condensation rates of the target elements on the phosphor film substrate. However, this method does not fully solve the problem of providing a stable phosphor film during display operation and at the same time providing a method that can be used for the economic deposition of phosphor films over large areas.
The Applicant's co-pending U.S. Pat. No. 6,793,782 discloses the sputtering of two targets to deposit a rare earth activated barium thioaluminate phosphor film. One of the sputtering targets comprises aluminum while the other sputtering target comprises europium doped barium sulfide. The sputtering is carried out in a low pressure atmosphere of hydrogen sulfide to provide sufficient sulfur content in the deposited film. The use of two sputtering targets facilitates modulation of the relative deposition rate of materials arising from each source which in turn facilitates deposition of a laminated film with a periodic composition alternately rich and poor in aluminum. The variation can be achieved by using a rotating or oscillating substrate that is alternately-positioned in the flux of atomic species sputtered from the respective targets. To the extent that the atomic flux from the two sources are spatially separated from one another, and to the extent that hydrogen sulfide is present in the sputtering chamber, a film can be deposited with a composition that is alternately aluminum sulfide and rare earth doped barium sulfide. The thickness of the layers can be altered by changing the rotation rate or the oscillation rate of the substrate. In this method, however, the composition modulation across the thickness of the deposited layer may be problematical for subsequent reaction of the deposited materials to form a homogeneous single phase phosphor material, since atomic species are required to diffuse within the deposited film to achieve a homogeneous composition on an atomic scale.
The Applicant's co-pending International Patent ApplicationPCT/CA03/01887 discloses the use of composite sputtering targets that can somewhat reduce the composition modulation problem discussed above. The composite sputtering target comprises an europium doped barium sulfide particle as an inclusion phase in an aluminum matrix. While such composite sputtering targets reduce the composition modulation problem, there may still remain some issues with respect to the stability of the barium sulfide when it is exposed to the ambient environment and the introduction of impurities such as oxygen or hydroxide into the deposited film due to sputtering of these species as impurities in the barium sulfide target with air and moisture. These problems have been addressed through the use of reactive sputtering using barium aluminum alloy targets in conjunction with aluminum targets in a hydrogen sulfide atmosphere as disclosed in Applicant's co-pending PCT/CA2005/000333.
Cerium and europium activated barium thiosilicate phosphors are known that exhibit blue photoluminescence and cathodoluminescence, however, these known phosphors have not been demonstrated to exhibit electroluminescence. More specifically, Ba2SiS4 and BaSi2S5 phosphors activated with cerium or europium (with ratios of silicon to barium of 2:1 or 1:2) have been suggested be useful as blue-emitting electroluminescent phosphor materials however, there is no data to support this hypothesis.
It is therefore desirable to develop methods for deposition of alternate thin film phosphor compositions for thick film dielectric electroluminescent displays where the compositions have lower CIE y coordinates for blue and that are also increasingly stable when in contact with the ambient environment during processing. This will enable a less stringent manufacturing environment leading to more cost effective manufacturing and to further obviate one or more of the disadvantages of the prior art methods.